Type inference by the compiler in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is type inference in Swift?

Type inference is when the Swift compiler automatically figures out the type of a variable or constant based on the value you assign to it, so you don't have to write the type explicitly.

beginner

How does Swift infer the type of <code>let number = 10</code>?

Swift sees the value 10 is an integer, so it infers the type of number as Int.

beginner

Can you give an example where type inference helps make code simpler?

Instead of writing <code>let name: String = "Alice"</code>, you can write <code>let name = "Alice"</code>. Swift infers <code>name</code> is a <code>String</code>.

intermediate

What happens if Swift cannot infer the type?

If Swift cannot figure out the type from the value, it will give an error and ask you to specify the type explicitly.

intermediate

Does type inference affect performance at runtime?

No, type inference happens only at compile time. The compiled program runs with explicit types, so there is no performance cost.

What type does Swift infer for let pi = 3.14?

A Double

B Int

C String

D Float

Swift infers decimal numbers like 3.14 as Double by default.

Which of these requires explicit type annotation in Swift?

A let count = 5

B let message = "Hello"

C let emptyArray = []

D let isActive = true

An empty array has no value to infer type from, so you must specify its type explicitly.

What is the benefit of type inference?

A Automatically fixes bugs

B Slows down the program

C Removes the need for variables

D Makes code shorter and easier to read

Type inference helps write less code and keeps it clear without losing type safety.

If you write let value = 42, what type does Swift assign to value?

A String

B Int

C Double

D Bool

42 is an integer, so Swift infers the type Int.

What happens if you try to assign a different type to a variable after type inference?

A Swift throws a compile-time error

B Swift allows it without error

C Swift changes the variable's type automatically

D Swift ignores the new value

Swift enforces type safety, so assigning a different type causes a compile-time error.

Explain what type inference is and why it is useful in Swift programming.

Think about how Swift guesses the type from the value you assign.

Describe a situation where you must specify the type explicitly even when using type inference.

When there is no value to guess the type from.